Packing containers intended for example for milk or other liquid or semi-liquid foodstuffs are usually manufactured from laminated flexible packing material which comprises layers of paper, thermoplastics and aluminium foil. A known packing container is formed in that a web of the said packing laminate, while being fed through the packing machine, is successively converted to tubular shape in that its two longitudinal edges are joined together and sealed to one another in a liquid-tight manner. Subsequently, contents are delivered in the required quantity to the lower end of the packing material tube which, with the help of co-operating sealing jaws, is then divided into individual packing containers separated from one another through repeated transverse sealings of the packing material tube and subsequent cutting. At the same time a certain forming process is carried out so that the finished packing containers obtain the desired, e.g. parallelepipedic, shape.
In the conversion of the lower end of the packing material tube to individual packing containers of e.g. substantially square cross-sectional area use is made of the contents and the gas which is present in the packing material tube as a hold-on in the forming process, since otherwise the flexible packing material wrinkles together in an irregular manner. To prevent the contents and above all the enclosed gas from being pressed out through the upper open end of the packing material tube during the forming work, a sealing unit is positioned in the packing material tube a short distance above the area where the transverse flattening and forming work of the material tube is taking place. The sealing unit is supported from inside the tube by a filling pipe for the contents extending vertically through the tube. Because the packing material tube as well as the filling pipe are mutually movable, great demands are made upon the construction of the sealing unit if the desired tightness is to be obtained during operation of the machine, and solutions available at present using rubber sleeves, flexible "skirts" and the like have proven unsatisfactory, mainly because the required tightness could not be maintained during relative lateral movements between the filling pipe and the packing material tube. Moreover, because the pressure of the bell-shaped sealing sleeves against the inside of the material tube has to be relatively high an undesirable wear of the seal becomes noticeable.